The present invention relates to the area of nuclear fuel materials mixed with a burnable poison for use in thermal reactors such as pressurized water (PWR) and boiling water (BWR) reactors. It is especially concerned with the mixtures of uranium oxide and gadolinium oxide.
In the past gadolinia (Gd.sub.2 O.sub.3) has been used extensively in BWR cores as a burnable absorber to control power distribution and reactivity. More recently it has also been used in PWR reactors. In the past gadolinia powder has been mixed with urania powder, then pressed and sintered into almost fully dense, solid cylindrical pellets. These poisoned pellets are then loaded in specific locations of a number of the fuel rods to provide the desired control over power distribution and reactivity.
Gadolinia has a number of drawbacks, especially as a burnable absorber for PWRs. It degrades the thermal properties of the fuel pellet by lowering the melting temperature and by lowering the thermal conductivity of the fuel pellet. In addition, gadolinium consists of a number of isotopes. The odd isotopes, gadolinium 155 and 157, have high thermal neutron absorption cross-sections and provide the desired control over power distribution and reactivity. These high cross-section isotopes however make up only about 30% of the gadolinium present (the remainder being low cross-section even isotopes) and burn out relatively quickly, transmuting upon absorption of a neutron to gadolinum 156 and 158, which, while having significantly lower cross-sections, parasitically absorb neutrons throughout the fuel cycle. The rapid burnout of gadolinium 155 and 157 produces an undersirable peaking in the power of the fuel rods containing the poisoned pellets, while the parasitic absorption of neutrons by the even isotopes produces an undesirable increase in the fuel cycle cost.
One of the ways the art has attempted to address some of the above problems, has been to propose a fuel pellet design consisting of an outer annular layer of only UO.sub.2 or UO.sub.2 +PuO.sub.2 and an inner plug of a mixture of UO.sub.2 and Gd.sub.2 O.sub.3. These and other similar designs are exemplified by Japanese Pat. No. 51-35885, Japanese Pat. No. Sho 54-150582 and Pat. No. Sho 59-187289. In these designs since the gadolinia content of the overall pellet has been concentrated in the center of the pellet a self shielding effect is produced which consequently slows the burnout of the absorber.
I now submit that rather surprisingly significant improvements can be obtained over the prior art pellet design by the improved poisoned fuel pellet design in accordance with the present invention.